Bug 37175 - NSUrlSession.CreateDownloadTask throws InvalidCastException on real device iPhone6, iOS9.2(13C75)
Summary: NSUrlSession.CreateDownloadTask throws InvalidCastException on real device iP...
Alias: None
Product: iOS
Classification: Xamarin
Component: Xamarin.iOS.dll (show other bugs)
Version: XI 9.4 (iOS 9.2)
Hardware: Macintosh Other
: --- normal
Target Milestone: (C9)
Assignee: Alex Soto [MSFT]
Depends on:
Reported: 2015-12-20 19:10 UTC by luc+xamarin
Modified: 2016-11-10 14:25 UTC (History)
5 users (show)

See Also:
Is this bug a regression?: ---
Last known good build:


Description luc+xamarin 2015-12-20 19:10:21 UTC

I am experiencing strange InvalidCastException in my application. 
This bug happens only on real device and was not reproduced in the emulator.
My device is iphone6 ios9.2 (13C75).

Would you please have a look, It feels like this bug is yours.
Let me know.


Please consider the following code :
var downloadmanager = new MyDownloadManager();

var downloadqueue = new NSOperationQueue {
	MaxConcurrentOperationCount = 1

string sessionidentifier = "myapp.download.images";
NSUrlSession nsurlsession = null;
using (var configuration = NSUrlSessionConfiguration.CreateBackgroundSessionConfiguration (sessionidentifier)) {
	nsurlsession = NSUrlSession.FromConfiguration (configuration, downloadmanager, downloadqueue);

foreach (var image in bigimages) { // about 2000 images around 100k each

   using (var url = NSUrl.FromString (image.Url))
   using (var request = NSUrlRequest.FromUrl (url)) {

      var task = NSUrlSessionDownloadTask (request); // throws InvalidCastException, after a few creation.


System.InvalidCastException: Unable to cast object of type 'Foundation.NSUrlSessionTask' to type 'Foundation.NSUrlSessionDownloadTask'
  at ObjCRuntime.Runtime.GetNSObject[T] (IntPtr ptr) <0x10015a650 + 0x00258> in <filename unknown>:0 
  at Foundation.NSUrlSession.CreateDownloadTask (Foundation.NSUrlRequest request) [0x0001c] in /Users/builder/data/lanes/2377/73229919/source/maccore/src/build/ios/native/Foundation/NSUrlSession.g.cs:240 
  at MyApp.MyDownloadManager.NSUrlSessionDownloadTask (Foundation.NSUrlRequest request) [0x00013] in ../MyApp/Iphone/MyApp/Sync/MyDownloadManager.cs:136 


=== Xamarin Studio ===

Version 5.10.1 (build 6)
Installation UUID: 32c1199d-cf1f-4098-ad19-5b592b046e78
	Mono 4.2.1 (explicit/6dd2d0d)
	GTK+ 2.24.23 (Raleigh theme)

	Package version: 402010102

=== Xamarin.Profiler ===

Location: /Applications/Xamarin Profiler.app/Contents/MacOS/Xamarin Profiler

=== Apple Developer Tools ===

Xcode 7.2 (9548)
Build 7C68

=== Xamarin.iOS ===

Version: (Business Edition)
Hash: 7322991
Branch: master
Build date: 2015-12-08 16:20:29-0500

=== Xamarin.Android ===

Version: (Starter Edition)
Android SDK: /Users/q/Library/Developer/Xamarin/android-sdk-macosx
	Supported Android versions:
		2.3   (API level 10)
		4.0.3 (API level 15)
		4.4   (API level 19)
		5.1   (API level 22)

SDK Tools Version: 24.1.2
SDK Platform Tools Version: 22
SDK Build Tools Version: 21.1.2

Java SDK: /usr
java version "1.7.0_71"
Java(TM) SE Runtime Environment (build 1.7.0_71-b14)
Java HotSpot(TM) 64-Bit Server VM (build 24.71-b01, mixed mode)

=== Xamarin Android Player ===

Not Installed

=== Xamarin.Mac ===

Not Installed

=== Build Information ===

Release ID: 510010006
Git revision: 0b60eecdb531933734519c13257d16a780274aab
Build date: 2015-12-04 20:28:20-05
Xamarin addins: 9876fd7c9837977178411ec7375b4352c0a0d6af
Build lane: monodevelop-lion-cycle6-baseline

=== Operating System ===

Mac OS X 10.11.2
Darwin n00b132 15.2.0 Darwin Kernel Version 15.2.0
    Fri Nov 13 19:56:56 PST 2015
    root:xnu-3248.20.55~2/RELEASE_X86_64 x86_64
Comment 1 luc+xamarin 2015-12-20 19:15:54 UTC
var task = NSUrlSessionDownloadTask (request); 
should be 
nsurlsession.CreateDownloadTask (request);

I use the following workaround until this bug gets fixed :

NSUrlSessionDownloadTask NSUrlSessionDownloadTask(NSUrlRequest request) {

   int bugcounter = 0;
   NSUrlSessionDownloadTask task = null;
   while (task == null) {
      try {
         Thread.Sleep (100);
         task = _session.CreateDownloadTask (request);
      } catch (InvalidCastException ) {
         // Xamarin.Ios bug ?
         if (bugcounter > 5) {
	   throw ;

   return task;
Comment 2 Rolf Bjarne Kvinge [MSFT] 2015-12-21 10:25:49 UTC
This is probably related to bug #24961, where the iOS API is returning objects of the wrong type.

You're calling this method on NSURLSession [1]:

- (NSURLSessionDownloadTask *)downloadTaskWithRequest:(NSURLRequest *)request

which clearly states that it will return an NSURLSessionDownloadTask instance, but yet it returns a NSURLSessionTask, which is a superclass (and thus not convertible to NSURLSessionDownloadTask).

This works fine in Objective-C, where there are no type checks, but we try to be type-safe, and that end up causing trouble in the binding when the API is lying to us.

[1] https://developer.apple.com/library/ios/documentation/Foundation/Reference/NSURLSession_class/#//apple_ref/occ/instm/NSURLSession/downloadTaskWithRequest:
Comment 3 luc+xamarin 2015-12-21 12:39:25 UTC
Dear Rolf, I do agree with your analysis.

Would you please fix this bug for the other NSURLSession classes as well :

NSURLSessionUploadTask   - Already fixed (as per bug #24961)
NSURLSessionDownloadTask - This bug report
NSURLSessionDataTask     - Does this one needs a fix also ?
NSURLSessionTask         - Shouldn't be affected.

Best regards,
Comment 4 Rolf Bjarne Kvinge [MSFT] 2016-01-13 17:55:39 UTC
@Luc, I have a potential fix, but I haven't been able to reproduce the InvalidCastException, so I don't know if the fix is complete or not (here's my test project: https://github.com/rolfbjarne/bug37175) - could you share your project with us so that I can verify my fix?
Comment 5 Alex Soto [MSFT] 2016-09-26 16:34:10 UTC
Fixed in xamarin-macios/master @ 912ab01d1eb68a668d058bcb67e7e3f088a1dd95
Comment 7 Naqeeb 2016-11-10 14:25:09 UTC
I have checked this issue with C9 build on physical device iOS 9.3.5 and observed that it is working fine. Here is the screencast for the same: http://www.screencast.com/t/KuIH78kd3

Environment info: https://gist.github.com/NaqeebAnsari/eb92b1cda2ad52a1cbce78e1cc30e6c5

Hence closing this issue.

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